1. Field of the Invention
The present invention relates to a processing circuit for an optical reproduction signal which converts the signal from a photo detector used in an optical disc apparatus etc. from a current to voltage (I-V) and outputs it divided into an RF signal and a servo signal.
2. Description of the Related Art
In an optical disc apparatus, the beam of laser light reflected at the optical disc is detected by, for example, a four division photo detector (PD) which detects the beams on four divided sections, and is used as the optical reproduction signal. This optical reproduction signal is usually treated as a current and includes a servo signal used for tracking servo control as a low frequency component and an RF signal used for reproduction of information as a high frequency component.
As a optical reproduction signal processing circuit for converting an optical reproduction signal from a current to voltage and outputting it divided into the servo signal and RF signal, the circuit shown in FIG. 1 is used. This circuit is equivalent to the so-called "separation type I" circuit disclosed in Signal Processing Technology in Optical Recording (in Japanese), Trikeps Co. (phonetic), p. 104, Feb. 20, 1989.
This optical reproduction signal processing circuit 7, as shown in FIGS. 2A to 2C, is used in the case where the low frequency component I.sub.LF for the RF signal and the high frequency component I.sub.SV for the servo signal do not overlap that much. The high frequency components including mainly the RF signal in the optical reproduction signals output from four photodiodes 340 to 346 are added by an RF signal I-V conversion circuit 70 comprised of capacitors C71 to C74, a resistor Rf70, and an amplification circuit AMP 70, converted from current to voltage, and then output as the RF signal.
On the other hand, the low frequency components including mainly the servo signal in the optical reproduction signals output from the four photodiodes 340 to 346 are converted from current to voltage by I-V conversion circuits 71 to 74 comprised respectively of resistors R71 to R74, resistors Rf71 to Rf74, and the amplification circuits AMP71 to AMP74 and then output as the servo signals SERVO1 to SERVO4.
By this, the optical reproduction signal with the frequency distributions of the servo signal component I.sub.SV and RF signal component I.sub.RF, shown in FIG. 2A for example is converted from a current to a voltage by the optical reproduction signal processing circuit 7. As a result, the RF signal (RF) shown by the graph of the frequency distribution in FIG. 2B and the servo signals (SERVO1 to SERVO4) shown by the graph of the frequency distribution in FIG. 2B are obtained.
As the recording density of optical discs becomes greater, however, the frequency distribution of the RF signal component and the frequency distribution of the servo signal component of the optical reproduction signal overlap more frequently. In this case, it is not possible to use the above-mentioned optical reproduction signal processing circuit 7.
Therefore, proposals have been made on improvements of the configuration of the optical signal processing circuit and improvements of the configuration of the optical system.
For example, as shown in FIG. 3, an optical reproduction signal processing circuit 8 of an example of the former improvement converts the optical reproduction signals from the photodiodes 340 to 346 from a current to voltage by the I-V conversion circuits 81 to 84, then adds the servo signals SERVO1 to SERVO4 by the addition circuit 80 and outputs the result as the RF signal. This circuit is equivalent to the operational amplifier type disclosed at page 103 of the aforementioned Signal Processing Technology in Optical Recording.
According to this optical reproduction signal processing circuit 8, the RF signal shown in FIG. 4A and the servo signal shown in FIG. 4B output from the optical reproduction signal processing circuit 8 do not have frequency characteristics. That is, the RF signal processing circuit and servo signal processing circuit disposed after the optical reproduction signal processing circuit 8 receive input signals derived from both of the RF signal component and servo signal component of the optical reproduction signal, and so can be used even in a case where the low frequency component used as the RF signal and the high frequency component used as the servo signal overlap.
The optical system of an example of the latter case, for example, as shown in FIG. 5, uses a half mirror 66 to divide into two the light reflected from the optical disc 3, separately detects the divided reflected light by the RF signal photo detector 62 and servo signal photo detector 64, and inputs the resultant optical reproduction signals to the RF signal preamplifier and servo signal preamplifier.
If such an optical system 9 is used, the RF signal and the servo signal are generated from the optical reproduction signals obtained from the separate photo detectors 62 and 64, so signal processing is possible without concern as to the overlap of the frequency distribution of the RF signal component and the frequency distribution of the servo signal component of the optical reproduction signals.
In the optical reproduction signal processing circuit 8 of the related art shown in FIG. 3, however, when use was made of a four division photo detector, it was necessary to add the output signals of the I-V conversion circuits 81 to 84 to obtain the RF signal. Accordingly, the power of the noise component of the I-V conversion circuits of the RF signal output of the optical reproduction signal processing circuit 8 became four times the noise component of the I-V conversion circuit of the RF signal of the optical reproduction signal processing circuit 7. To reduce the noise component, it was necessary to use an amplification circuit with low noise.
In addition, the RF signal output and the servo signal output of the optical reproduction signal processing circuit 8 extended over a wide frequency region from DC to several tens of MHz, so the amplification circuits had to have broader coverage and DC stability.
In this way, the amplification circuits used for the optical reproduction signal processing circuit 8 had to be higher class circuits meeting the above conditions and therefore it suffers from the disadvantage that the cost of the optical disc apparatus using the optical reproduction signal processing circuit 8 became higher.
The above disadvantage in the I-V conversion processing can be solved by using the optical system 9 shown in FIG. 5, but in this type of optical system 9, it was necessary to further provide a half mirror 66 and photo detectors in the optical system in the case of using the optical reproduction signal processing circuits 7 and 8 of FIG. 1 and FIG. 3. As a result, the cost of the optical disc apparatus using the optical system 9 increased by the amount of the half mirror 66 and the photo detectors and efforts to reduce the size of the apparatus were obstructed.